3.18.58 x86 Options

These ‘-m’ options are defined for the x86 family of computers.

-march=cpu-type

Generate instructions for the machine type cpu-type. In contrast to
-mtune=cpu-type, which merely tunes the generated code
for the specified cpu-type, -march=cpu-type allows GCC
to generate code that may not run at all on processors other than the one
indicated. Specifying -march=cpu-type implies
-mtune=cpu-type.

The choices for cpu-type are:

‘native’

This selects the CPU to generate code for at compilation time by determining
the processor type of the compiling machine. Using -march=native
enables all instruction subsets supported by the local machine (hence
the result might not run on different machines). Using -mtune=native
produces code optimized for the local machine under the constraints
of the selected instruction set.

‘x86-64’

A generic CPU with 64-bit extensions.

‘i386’

Original Intel i386 CPU.

‘i486’

Intel i486 CPU. (No scheduling is implemented for this chip.)

‘i586’

‘pentium’

Intel Pentium CPU with no MMX support.

‘lakemont’

Intel Lakemont MCU, based on Intel Pentium CPU.

‘pentium-mmx’

Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.

‘pentiumpro’

Intel Pentium Pro CPU.

‘i686’

When used with -march, the Pentium Pro
instruction set is used, so the code runs on all i686 family chips.
When used with -mtune, it has the same meaning as ‘generic’.

Tune to cpu-type everything applicable about the generated code, except
for the ABI and the set of available instructions.
While picking a specific cpu-type schedules things appropriately
for that particular chip, the compiler does not generate any code that
cannot run on the default machine type unless you use a
-march=cpu-type option.
For example, if GCC is configured for i686-pc-linux-gnu
then -mtune=pentium4 generates code that is tuned for Pentium 4
but still runs on i686 machines.

The choices for cpu-type are the same as for -march.
In addition, -mtune supports 2 extra choices for cpu-type:

‘generic’

Produce code optimized for the most common IA32/AMD64/EM64T processors.
If you know the CPU on which your code will run, then you should use
the corresponding -mtune or -march option instead of
-mtune=generic. But, if you do not know exactly what CPU users
of your application will have, then you should use this option.

As new processors are deployed in the marketplace, the behavior of this
option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect
the processors
that are most common at the time that version of GCC is released.

There is no -march=generic option because -march
indicates the instruction set the compiler can use, and there is no
generic instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.

‘intel’

Produce code optimized for the most current Intel processors, which are
Haswell and Silvermont for this version of GCC. If you know the CPU
on which your code will run, then you should use the corresponding
-mtune or -march option instead of -mtune=intel.
But, if you want your application performs better on both Haswell and
Silvermont, then you should use this option.

As new Intel processors are deployed in the marketplace, the behavior of
this option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect
the most current Intel processors at the time that version of GCC is
released.

There is no -march=intel option because -march indicates
the instruction set the compiler can use, and there is no common
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.

Use the standard 387 floating-point coprocessor present on the majority of chips and
emulated otherwise. Code compiled with this option runs almost everywhere.
The temporary results are computed in 80-bit precision instead of the precision
specified by the type, resulting in slightly different results compared to most
of other chips. See -ffloat-store for more detailed description.

This is the default choice for non-Darwin x86-32 targets.

‘sse’

Use scalar floating-point instructions present in the SSE instruction set.
This instruction set is supported by Pentium III and newer chips,
and in the AMD line
by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE
instruction set supports only single-precision arithmetic, thus the double and
extended-precision arithmetic are still done using 387. A later version, present
only in Pentium 4 and AMD x86-64 chips, supports double-precision
arithmetic too.

For the x86-32 compiler, you must use -march=cpu-type, -msse
or -msse2 switches to enable SSE extensions and make this option
effective. For the x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid
the numerical instability problems of 387 code, but may break some existing
code that expects temporaries to be 80 bits.

This is the default choice for the x86-64 compiler, Darwin x86-32 targets,
and the default choice for x86-32 targets with the SSE2 instruction set
when -ffast-math is enabled.

‘sse,387’

‘sse+387’

‘both’

Attempt to utilize both instruction sets at once. This effectively doubles the
amount of available registers, and on chips with separate execution units for
387 and SSE the execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does not model separate
functional units well, resulting in unstable performance.

-masm=dialect

Output assembly instructions using selected dialect. Also affects
which dialect is used for basic asm (see Basic Asm) and
extended asm (see Extended Asm). Supported choices (in dialect
order) are ‘att’ or ‘intel’. The default is ‘att’. Darwin does
not support ‘intel’.

-mieee-fp

-mno-ieee-fp

Control whether or not the compiler uses IEEE floating-point
comparisons. These correctly handle the case where the result of a
comparison is unordered.

-m80387

-mhard-float

Generate output containing 80387 instructions for floating point.

-mno-80387

-msoft-float

Generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for
cross-compilation.

On machines where a function returns floating-point results in the 80387
register stack, some floating-point opcodes may be emitted even if
-msoft-float is used.

-mno-fp-ret-in-387

Do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types
float and double in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate
an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned
in ordinary CPU registers instead.

-mno-fancy-math-387

Some 387 emulators do not support the sin, cos and
sqrt instructions for the 387. Specify this option to avoid
generating those instructions.
This option is overridden when -march
indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.

-malign-double

-mno-align-double

Control whether GCC aligns double, long double, and
long long variables on a two-word boundary or a one-word
boundary. Aligning double variables on a two-word boundary
produces code that runs somewhat faster on a Pentium at the
expense of more memory.

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch,
structures containing the above types are aligned differently than
the published application binary interface specifications for the x86-32
and are not binary compatible with structures in code compiled
without that switch.

-m96bit-long-double

-m128bit-long-double

These switches control the size of long double type. The x86-32
application binary interface specifies the size to be 96 bits,
so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer) prefer long double
to be aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns long double
to a 16-byte boundary by padding the long double with an additional
32-bit zero.

In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that long double is aligned on 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a long double.

Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing long double variables,
as well as modifying the function calling convention for functions taking
long double. Hence they are not binary-compatible
with code compiled without that switch.

-mlong-double-64

-mlong-double-80

-mlong-double-128

These switches control the size of long double type. A size
of 64 bits makes the long double type equivalent to the double
type. This is the default for 32-bit Bionic C library. A size
of 128 bits makes the long double type equivalent to the
__float128 type. This is the default for 64-bit Bionic C library.

Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing long double variables,
as well as modifying the function calling convention for functions taking
long double. Hence they are not binary-compatible
with code compiled without that switch.

-malign-data=type

Control how GCC aligns variables. Supported values for type are
‘compat’ uses increased alignment value compatible uses GCC 4.8
and earlier, ‘abi’ uses alignment value as specified by the
psABI, and ‘cacheline’ uses increased alignment value to match
the cache line size. ‘compat’ is the default.

-mlarge-data-threshold=threshold

When -mcmodel=medium is specified, data objects larger than
threshold are placed in the large data section. This value must be the
same across all objects linked into the binary, and defaults to 65535.

-mrtd

Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the ret num
instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.

You can specify that an individual function is called with this calling
sequence with the function attribute stdcall. You can also
override the -mrtd option by using the function attribute
cdecl. See Function Attributes.

Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code is generated for calls to those
functions.

In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)

-mregparm=num

Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a specific
function by using the function attribute regparm.
See Function Attributes.

Warning: if you use this switch, and
num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and
startup modules.

-msseregparm

Use SSE register passing conventions for float and double arguments
and return values. You can control this behavior for a specific
function by using the function attribute sseregparm.
See Function Attributes.

Warning: if you use this switch then you must build all
modules with the same value, including any libraries. This includes
the system libraries and startup modules.

-mvect8-ret-in-mem

Return 8-byte vectors in memory instead of MMX registers. This is the
default on Solaris 8 and 9 and VxWorks to match the ABI of the Sun
Studio compilers until version 12. Later compiler versions (starting
with Studio 12 Update 1) follow the ABI used by other x86 targets, which
is the default on Solaris 10 and later. Only use this option if
you need to remain compatible with existing code produced by those
previous compiler versions or older versions of GCC.

-mpc32

-mpc64

-mpc80

Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32
is specified, the significands of results of floating-point operations are
rounded to 24 bits (single precision); -mpc64 rounds the
significands of results of floating-point operations to 53 bits (double
precision) and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double precision), which is
the default. When this option is used, floating-point operations in higher
precisions are not available to the programmer without setting the FPU
control word explicitly.

Setting the rounding of floating-point operations to less than the default
80 bits can speed some programs by 2% or more. Note that some mathematical
libraries assume that extended-precision (80-bit) floating-point operations
are enabled by default; routines in such libraries could suffer significant
loss of accuracy, typically through so-called “catastrophic cancellation”,
when this option is used to set the precision to less than extended precision.

-mstackrealign

Realign the stack at entry. On the x86, the -mstackrealign
option generates an alternate prologue and epilogue that realigns the
run-time stack if necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep 16-byte stack alignment for
SSE compatibility. See also the attribute force_align_arg_pointer,
applicable to individual functions.

-mpreferred-stack-boundary=num

Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified,
the default is 4 (16 bytes or 128 bits).

Warning: When generating code for the x86-64 architecture with
SSE extensions disabled, -mpreferred-stack-boundary=3 can be
used to keep the stack boundary aligned to 8 byte boundary. Since
x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and
intended to be used in controlled environment where stack space is
important limitation. This option leads to wrong code when functions
compiled with 16 byte stack alignment (such as functions from a standard
library) are called with misaligned stack. In this case, SSE
instructions may lead to misaligned memory access traps. In addition,
variable arguments are handled incorrectly for 16 byte aligned
objects (including x87 long double and __int128), leading to wrong
results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This
includes the system libraries and startup modules.

-mincoming-stack-boundary=num

Assume the incoming stack is aligned to a 2 raised to num byte
boundary. If -mincoming-stack-boundary is not specified,
the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values
should be aligned to an 8-byte boundary (see -malign-double) or
suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type __m128 may not work
properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred
stack boundary from a function compiled with a lower preferred stack
boundary most likely misaligns the stack. It is recommended that
libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage, such
as embedded systems and operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.

GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx instructions
when needed.

These options enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications that
perform run-time CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In particular,
the file containing the CPU detection code should be compiled without
these options.

-mdump-tune-features

This option instructs GCC to dump the names of the x86 performance
tuning features and default settings. The names can be used in
-mtune-ctrl=feature-list.

-mtune-ctrl=feature-list

This option is used to do fine grain control of x86 code generation features.
feature-list is a comma separated list of feature names. See also
-mdump-tune-features. When specified, the feature is turned
on if it is not preceded with ‘^’, otherwise, it is turned off.
-mtune-ctrl=feature-list is intended to be used by GCC
developers. Using it may lead to code paths not covered by testing and can
potentially result in compiler ICEs or runtime errors.

-mno-default

This option instructs GCC to turn off all tunable features. See also
-mtune-ctrl=feature-list and -mdump-tune-features.

-mcld

This option instructs GCC to emit a cld instruction in the prologue
of functions that use string instructions. String instructions depend on
the DF flag to select between autoincrement or autodecrement mode. While the
ABI specifies the DF flag to be cleared on function entry, some operating
systems violate this specification by not clearing the DF flag in their
exception dispatchers. The exception handler can be invoked with the DF flag
set, which leads to wrong direction mode when string instructions are used.
This option can be enabled by default on 32-bit x86 targets by configuring
GCC with the --enable-cld configure option. Generation of cld
instructions can be suppressed with the -mno-cld compiler option
in this case.

-mvzeroupper

This option instructs GCC to emit a vzeroupper instruction
before a transfer of control flow out of the function to minimize
the AVX to SSE transition penalty as well as remove unnecessary zeroupper
intrinsics.

-mprefer-avx128

This option instructs GCC to use 128-bit AVX instructions instead of
256-bit AVX instructions in the auto-vectorizer.

-mprefer-vector-width=opt

This option instructs GCC to use opt-bit vector width in instructions
instead of default on the selected platform.

‘none’

No extra limitations applied to GCC other than defined by the selected platform.

‘128’

Prefer 128-bit vector width for instructions.

‘256’

Prefer 256-bit vector width for instructions.

‘512’

Prefer 512-bit vector width for instructions.

-mcx16

This option enables GCC to generate CMPXCHG16B instructions in 64-bit
code to implement compare-and-exchange operations on 16-byte aligned 128-bit
objects. This is useful for atomic updates of data structures exceeding one
machine word in size. The compiler uses this instruction to implement
__sync Builtins. However, for __atomic Builtins operating on
128-bit integers, a library call is always used.

-msahf

This option enables generation of SAHF instructions in 64-bit code.
Early Intel Pentium 4 CPUs with Intel 64 support,
prior to the introduction of Pentium 4 G1 step in December 2005,
lacked the LAHF and SAHF instructions
which are supported by AMD64.
These are load and store instructions, respectively, for certain status flags.
In 64-bit mode, the SAHF instruction is used to optimize fmod,
drem, and remainder built-in functions;
see Other Builtins for details.

-mmovbe

This option enables use of the movbe instruction to implement
__builtin_bswap32 and __builtin_bswap64.

This option enables use of RCPSS and RSQRTSS instructions
(and their vectorized variants RCPPS and RSQRTPS)
with an additional Newton-Raphson step
to increase precision instead of DIVSS and SQRTSS
(and their vectorized
variants) for single-precision floating-point arguments. These instructions
are generated only when -funsafe-math-optimizations is enabled
together with -ffinite-math-only and -fno-trapping-math.
Note that while the throughput of the sequence is higher than the throughput
of the non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).

This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which may
be preceded by a ‘!’ to invert the option:

‘all’

Enable all estimate instructions.

‘default’

Enable the default instructions, equivalent to -mrecip.

‘none’

Disable all estimate instructions, equivalent to -mno-recip.

‘div’

Enable the approximation for scalar division.

‘vec-div’

Enable the approximation for vectorized division.

‘sqrt’

Enable the approximation for scalar square root.

‘vec-sqrt’

Enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables
all of the reciprocal approximations, except for square root.

-mveclibabi=type

Specifies the ABI type to use for vectorizing intrinsics using an
external library. Supported values for type are ‘svml’
for the Intel short
vector math library and ‘acml’ for the AMD math core library.
To use this option, both -ftree-vectorize and
-funsafe-math-optimizations have to be enabled, and an SVML or ACML
ABI-compatible library must be specified at link time.

Generate code for the specified calling convention. Permissible values
are ‘sysv’ for the ABI used on GNU/Linux and other systems, and
‘ms’ for the Microsoft ABI. The default is to use the Microsoft
ABI when targeting Microsoft Windows and the SysV ABI on all other systems.
You can control this behavior for specific functions by
using the function attributes ms_abi and sysv_abi.
See Function Attributes.

-mforce-indirect-call

Force all calls to functions to be indirect. This is useful
when using Intel Processor Trace where it generates more precise timing
information for function calls.

-mmanual-endbr

Insert ENDBR instruction at function entry only via the cf_check
function attribute. This is useful when used with the option
-fcf-protection=branch to control ENDBR insertion at the
function entry.

-mcall-ms2sysv-xlogues

Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a
System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By
default, the code for saving and restoring these registers is emitted inline,
resulting in fairly lengthy prologues and epilogues. Using
-mcall-ms2sysv-xlogues emits prologues and epilogues that
use stubs in the static portion of libgcc to perform these saves and restores,
thus reducing function size at the cost of a few extra instructions.

-mtls-dialect=type

Generate code to access thread-local storage using the ‘gnu’ or
‘gnu2’ conventions. ‘gnu’ is the conservative default;
‘gnu2’ is more efficient, but it may add compile- and run-time
requirements that cannot be satisfied on all systems.

-mpush-args

-mno-push-args

Use PUSH operations to store outgoing parameters. This method is shorter
and usually equally fast as method using SUB/MOV operations and is enabled
by default. In some cases disabling it may improve performance because of
improved scheduling and reduced dependencies.

-maccumulate-outgoing-args

If enabled, the maximum amount of space required for outgoing arguments is
computed in the function prologue. This is faster on most modern CPUs
because of reduced dependencies, improved scheduling and reduced stack usage
when the preferred stack boundary is not equal to 2. The drawback is a notable
increase in code size. This switch implies -mno-push-args.

-mthreads

Support thread-safe exception handling on MinGW. Programs that rely
on thread-safe exception handling must compile and link all code with the
-mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per-thread exception-handling data.

If packed is used on a structure, or if bit-fields are used,
it may be that the Microsoft ABI lays out the structure differently
than the way GCC normally does. Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.

This option is enabled by default for Microsoft Windows
targets. This behavior can also be controlled locally by use of variable
or type attributes. For more information, see x86 Variable Attributes
and x86 Type Attributes.

The Microsoft structure layout algorithm is fairly simple with the exception
of the bit-field packing.
The padding and alignment of members of structures and whether a bit-field
can straddle a storage-unit boundary are determine by these rules:

Structure members are stored sequentially in the order in which they are
declared: the first member has the lowest memory address and the last member
the highest.

Every data object has an alignment requirement. The alignment requirement
for all data except structures, unions, and arrays is either the size of the
object or the current packing size (specified with either the
aligned attribute or the pack pragma),
whichever is less. For structures, unions, and arrays,
the alignment requirement is the largest alignment requirement of its members.
Every object is allocated an offset so that:

offset % alignment_requirement == 0

Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation
unit if the integral types are the same size and if the next bit-field fits
into the current allocation unit without crossing the boundary imposed by the
common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.

The size of t1 is 8 bytes with the zero-length bit-field. If the
zero-length bit-field were removed, t1’s size would be 4 bytes.

If a zero-length bit-field is inserted after a bit-field, foo, and the
alignment of the zero-length bit-field is greater than the member that follows it,
bar, bar is aligned as the type of the zero-length bit-field.

For t2, bar is placed at offset 2, rather than offset 1.
Accordingly, the size of t2 is 4. For t3, the zero-length
bit-field does not affect the alignment of bar or, as a result, the size
of the structure.

Taking this into account, it is important to note the following:

If a zero-length bit-field follows a normal bit-field, the type of the
zero-length bit-field may affect the alignment of the structure as whole. For
example, t2 has a size of 4 bytes, since the zero-length bit-field follows a
normal bit-field, and is of type short.

Even if a zero-length bit-field is not followed by a normal bit-field, it may
still affect the alignment of the structure:

struct
{
char foo : 6;
long : 0;
} t4;

Here, t4 takes up 4 bytes.

Zero-length bit-fields following non-bit-field members are ignored:

struct
{
char foo;
long : 0;
char bar;
} t5;

Here, t5 takes up 2 bytes.

-mno-align-stringops

Do not align the destination of inlined string operations. This switch reduces
code size and improves performance in case the destination is already aligned,
but GCC doesn’t know about it.

-minline-all-stringops

By default GCC inlines string operations only when the destination is
known to be aligned to least a 4-byte boundary.
This enables more inlining and increases code
size, but may improve performance of code that depends on fast
memcpy and memset for short lengths.
The option enables inline expansion of strlen for all
pointer alignments.

-minline-stringops-dynamically

For string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large blocks.

-mstringop-strategy=alg

Override the internal decision heuristic for the particular algorithm to use
for inlining string operations. The allowed values for alg are:

‘rep_byte’

‘rep_4byte’

‘rep_8byte’

Expand using i386 rep prefix of the specified size.

‘byte_loop’

‘loop’

‘unrolled_loop’

Expand into an inline loop.

‘libcall’

Always use a library call.

-mmemcpy-strategy=strategy

Override the internal decision heuristic to decide if __builtin_memcpy
should be inlined and what inline algorithm to use when the expected size
of the copy operation is known. strategy
is a comma-separated list of alg:max_size:dest_align triplets.
alg is specified in -mstringop-strategy, max_size specifies
the max byte size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be -1. The max_size of the triplets
in the list must be specified in increasing order. The minimal byte size for
alg is 0 for the first triplet and max_size + 1 of the
preceding range.

-mmemset-strategy=strategy

The option is similar to -mmemcpy-strategy= except that it is to control
__builtin_memset expansion.

-momit-leaf-frame-pointer

Don’t keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up, and restore frame pointers and
makes an extra register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for leaf functions,
which might make debugging harder.

-mtls-direct-seg-refs

-mno-tls-direct-seg-refs

Controls whether TLS variables may be accessed with offsets from the
TLS segment register (%gs for 32-bit, %fs for 64-bit),
or whether the thread base pointer must be added. Whether or not this
is valid depends on the operating system, and whether it maps the
segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on.

-msse2avx

-mno-sse2avx

Specify that the assembler should encode SSE instructions with VEX
prefix. The option -mavx turns this on by default.

-mfentry

-mno-fentry

If profiling is active (-pg), put the profiling
counter call before the prologue.
Note: On x86 architectures the attribute ms_hook_prologue
isn’t possible at the moment for -mfentry and -pg.

-mrecord-mcount

-mno-record-mcount

If profiling is active (-pg), generate a __mcount_loc section
that contains pointers to each profiling call. This is useful for
automatically patching and out calls.

-mnop-mcount

-mno-nop-mcount

If profiling is active (-pg), generate the calls to
the profiling functions as NOPs. This is useful when they
should be patched in later dynamically. This is likely only
useful together with -mrecord-mcount.

-minstrument-return=type

Instrument function exit in -pg -mfentry instrumented functions with
call to specified function. This only instruments true returns ending
with ret, but not sibling calls ending with jump. Valid types
are none to not instrument, call to generate a call to __return__,
or nop5 to generate a 5 byte nop.

-mrecord-return

-mno-record-return

Generate a __return_loc section pointing to all return instrumentation code.

-mfentry-name=name

Set name of __fentry__ symbol called at function entry for -pg -mfentry functions.

-mfentry-section=name

Set name of section to record -mrecord-mcount calls (default __mcount_loc).

-mskip-rax-setup

-mno-skip-rax-setup

When generating code for the x86-64 architecture with SSE extensions
disabled, -mskip-rax-setup can be used to skip setting up RAX
register when there are no variable arguments passed in vector registers.

Warning: Since RAX register is used to avoid unnecessarily
saving vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space,
misbehave or jump to a random location. GCC 4.4 or newer don’t have
those issues, regardless the RAX register value.

-m8bit-idiv

-mno-8bit-idiv

On some processors, like Intel Atom, 8-bit unsigned integer divide is
much faster than 32-bit/64-bit integer divide. This option generates a
run-time check. If both dividend and divisor are within range of 0
to 255, 8-bit unsigned integer divide is used instead of
32-bit/64-bit integer divide.

-mavx256-split-unaligned-load

-mavx256-split-unaligned-store

Split 32-byte AVX unaligned load and store.

-mstack-protector-guard=guard

-mstack-protector-guard-reg=reg

-mstack-protector-guard-offset=offset

Generate stack protection code using canary at guard. Supported
locations are ‘global’ for global canary or ‘tls’ for per-thread
canary in the TLS block (the default). This option has effect only when
-fstack-protector or -fstack-protector-all is specified.

With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify
which segment register (%fs or %gs) to use as base register
for reading the canary, and from what offset from that base register.
The default for those is as specified in the relevant ABI.

-mgeneral-regs-only

Generate code that uses only the general-purpose registers. This
prevents the compiler from using floating-point, vector, mask and bound
registers.

-mindirect-branch=choice

Convert indirect call and jump with choice. The default is
‘keep’, which keeps indirect call and jump unmodified.
‘thunk’ converts indirect call and jump to call and return thunk.
‘thunk-inline’ converts indirect call and jump to inlined call
and return thunk. ‘thunk-extern’ converts indirect call and jump
to external call and return thunk provided in a separate object file.
You can control this behavior for a specific function by using the
function attribute indirect_branch. See Function Attributes.

Note that -mcmodel=large is incompatible with
-mindirect-branch=thunk and
-mindirect-branch=thunk-extern since the thunk function may
not be reachable in the large code model.

Note that -mindirect-branch=thunk-extern is incompatible with
-fcf-protection=branch since the external thunk cannot be modified
to disable control-flow check.

-mfunction-return=choice

Convert function return with choice. The default is ‘keep’,
which keeps function return unmodified. ‘thunk’ converts function
return to call and return thunk. ‘thunk-inline’ converts function
return to inlined call and return thunk. ‘thunk-extern’ converts
function return to external call and return thunk provided in a separate
object file. You can control this behavior for a specific function by
using the function attribute function_return.
See Function Attributes.

Note that -mcmodel=large is incompatible with
-mfunction-return=thunk and
-mfunction-return=thunk-extern since the thunk function may
not be reachable in the large code model.

-mindirect-branch-register

Force indirect call and jump via register.

These ‘-m’ switches are supported in addition to the above
on x86-64 processors in 64-bit environments.

-m32

-m64

-mx32

-m16

-miamcu

Generate code for a 16-bit, 32-bit or 64-bit environment.
The -m32 option sets int, long, and pointer types
to 32 bits, and
generates code that runs on any i386 system.

The -m64 option sets int to 32 bits and long and pointer
types to 64 bits, and generates code for the x86-64 architecture.
For Darwin only the -m64 option also turns off the -fno-pic
and -mdynamic-no-pic options.

The -mx32 option sets int, long, and pointer types
to 32 bits, and
generates code for the x86-64 architecture.

The -m16 option is the same as -m32, except for that
it outputs the .code16gcc assembly directive at the beginning of
the assembly output so that the binary can run in 16-bit mode.

The -miamcu option generates code which conforms to Intel MCU
psABI. It requires the -m32 option to be turned on.

-mno-red-zone

Do not use a so-called “red zone” for x86-64 code. The red zone is mandated
by the x86-64 ABI; it is a 128-byte area beyond the location of the
stack pointer that is not modified by signal or interrupt handlers
and therefore can be used for temporary data without adjusting the stack
pointer. The flag -mno-red-zone disables this red zone.

-mcmodel=small

Generate code for the small code model: the program and its symbols must
be linked in the lower 2 GB of the address space. Pointers are 64 bits.
Programs can be statically or dynamically linked. This is the default
code model.

-mcmodel=kernel

Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space.
This model has to be used for Linux kernel code.

-mcmodel=medium

Generate code for the medium model: the program is linked in the lower 2
GB of the address space. Small symbols are also placed there. Symbols
with sizes larger than -mlarge-data-threshold are put into
large data or BSS sections and can be located above 2GB. Programs can
be statically or dynamically linked.

-mcmodel=large

Generate code for the large model. This model makes no assumptions
about addresses and sizes of sections.

-maddress-mode=long

Generate code for long address mode. This is only supported for 64-bit
and x32 environments. It is the default address mode for 64-bit
environments.

-maddress-mode=short

Generate code for short address mode. This is only supported for 32-bit
and x32 environments. It is the default address mode for 32-bit and
x32 environments.